Page 82 - 4571

constituent of the upper mantle. An important aspect of the chemical composition of
igneous rocks is silica (SiO 2) content. Typically, the silica content of crustal rocks
ranges from a low of about 40% in ultramafic rocks to a high of more than 70% in
granitic rocks. The amount of silica present in magma strongly influences its behavior.
Granitic magma, which has a high silica content, is quite viscous (“thick”) and may
erupt at temperatures as low as 700 °C. On the other hand, basaltic magmas are low in
silica and are generally more fluid.

Task 2. Make up a list of all the names of rocks mentioned in the text.
Provide the transcription of the words.

Task 3. Answer the questions:
1. Which category of rocks is associated with volcanic activity?
2. Characterize peridotite. Why is its chemical composition referred to as
ultramafic?
3. In what way does the amount of silica present in magma strongly influence its
behaviour?

Test yourself

Task 1. Tell whether the sentences are true or false. Correct the false ones.
1. Lava is molten rock which reaches Earth’s surface.
2. Magma is completely or partly molten rock, which on cooling solidifies to form
an igneous rock composed of silicate minerals.
3. Most magmas consist of two distinct parts—a solid component and a gaseous
phase.
4. The solid components in magma are nonsilicate minerals.
5. When magma loses its mobility before reaching the surface it crystallizes to form
plutonic rocks.
6. Igneous rocks that form when molten rock solidifies at the surface are called
volcanic rocks.
7. Igneous rocks are rarely composed of silicate minerals.
8. Olivine, pyroxene, amphibole, and biotite mica are the common light silicate
minerals of Earth’s crust.
9. The light silicates include quartz, muscovite mica, and the feldspars.
10. An important aspect of the chemical composition of igneous rocks is silica
content.

Task 2. Using the information from the previous text and analysing Figure
8.1 try to complete the summary of the topic “The Nature of Magma. Igneous
Processes. Igneous Compositions. Other Compositional Groups”.

In summary, igneous rocks can be divided into broad groups according to the
proportions of …and …minerals they contain. …(felsic) rocks, which are composed
almost entirely of the lightcolored minerals quartz and feldspar, are at one end of the
compositional spectrum. …(mafic) rocks,which contain abundant dark silicate minerals
in addition to plagioclase feldspar, make up the other major igneous rock group of
Earth’s crust. Between these groups are rocks with an …(andesitic) composition.
…rocks, which lack light-colored minerals, lie at the far end of the compositional
spectrum from granitic rocks.

81